The present invention relates generally to the recovery of silver from spent photographic fixer solutions and, more particularly, to a method for treating such solutions for effecting silver recovery and the conditioning of the remaining effluent for discharge into a sewage system.
Spent photographic fixer solutions contain up to about 6 gms/liter of dissolved silver along with relatively high concentrations of silver complexing agents such as thiosulfate and sulfite ions and other ions of such as halides, acetates, ammonium, and aluminum. Various state and federal regulations severely restrict the discharge of spent photographic fixer solutions into sewage systems or the environment because of the silver content and the excess level in biological and chemical oxygen demand due to the presence of oxidizable ions such as thiosulfate and sulfite. A number of silver recovery and effluent conditioning procedures have been reported in the literature and these procedures generally fall into one of three broad categories such as electrochemical, ion exchange or precipitation techniques. Attempts to commercialize these methods, however, have revealed a wide variety of economic or technical problems which render them not readily amenable for use in large-scale silver recovery and effluent conditioning as required for photoreproduction plants. For example, such problems include prolonged reaction times, insoluble sludge formation, the use of expensive or undesirable additives and increased cost for mainline or auxiliary process equipment. Also, these prior art techniques often fail to achieve the intended degree of silver recovery thereby producing undesirable effluents that may present a toxic solid waste disposal problem.